Vegetation and soil

How might climate change effect ecosystem distribution in the UK? What impact will climate change have on biodiversity in the UK? How will climate change modify soil distribution patterns for the UK?

Ecosystem Function and Productivity

An ecosystem is a functioning and interacting system composed of plants, animals and other living organisms (the biomass) and the surrounding environment to which they are linked physically, chemically and biologically. Ecosystems are open systems, with their continued functioning dependent upon external inputs of solar energy and materials such as water and dissolved nutrients. The annual rate of biomass growth in an ecosystem is termed net primary productivity (NPP).

The inter-relatedness of the component parts of an ecosystem are apparent in the study of (i) the flows of energy that occur along food chains and within food webs and (ii) the nutrient cycles that maintain plant and animal growth over time. Food chains are sustained by sunlight, captured by the leaves of green plants and converted into carbohydrates as part of the process of photosynthesis. Nutrient cycles are fed by inputs of dissolved minerals in rainfall, countering losses from leaching and run-off.

Ecosystems, food chains and nutrient cycles are all said to exist in a state of dynamic equilibrium, where inputs of energy and matter are balanced with outputs in the long term. However this assumes relatively constant inputs of solar radiation and precipitation (and outputs such as evaporation). Climate change means that these inputs are changing.

How will climate change modify the UK's ecosystems?

There is no firm consensus from scientists on exactly how climate will change in the future and the consequent impact that this will have on photosynthesis and nutrient cycling. However, data suggests that:

Global atmospheric concentration of CO2 has increased to a current level of 380 parts per million.

Growing seasons for plants in central England have already lengthened by about one month since 1900.

Heat-waves have become more frequent in summer (e.g. 2006), while there are now fewer frosts and winter cold spells.

Wetter winters have arisen relative to summers throughout the UK over the last 200 years but with a larger proportion of precipitation falling on heavy rainfall days (causing surface run-off, meaning that less water soaks into the soil).

This wide range of climatic changes means that impacts on productivity, food webs and nutrient cycles are complex and often contradictory. They are neither uniformly negative nor positive across the whole of the UK. For instance, growth rates may be increasing wherever plants are not limited by a lack of water or nutrient availability - for instance on flood plains with a high water table. This is due to increased photosynthesis due to a longer thermal growing season and increased levels of atmospheric CO2.

However, where water supplies are limited - for instance in well-drained areas with permeable soils or parent material - then trees will become stressed and experience leaf fall during heat-waves. Grasses will die back to their roots. This leads to a decline in biomass and productivity within the autotrophic layer (and in turn will adversely effect other organisms in the food chain).

Student Practice Question:

Outline the factors that influence rates of ecosystem productivity

As well as looking at how rates vary from place-to-place, a good answer might describe climate change as a factor that may have a long-term impact on rates of productivity over time.

Ecosystem Distribution

Biomes are ecosystems that extend over large geographical areas and take their name from the dominant vegetation type within (for example, coniferous forest). A biome may cover a large area of a continent or may straddle several (the tropical rainforest is found in three). Each has its own characteristic soil, flora and fauna. Climate determines the distribution pattern of the different biomes. If climate is changing, then it stands to reason that the map of world biomes will also need to be modified.

The global map of biomes shows the British Isles as a zone of Temperate Deciduous Forest. In the absence of local arresting factors to plant succession, this is the normal climatic climax community. Deciduous trees are favoured by the UK's moist maritime (coastal) climate. However, at a local scale the situation is far more complex. Improved or acid grassland, heather moorland, coniferous forest, marshland and alpine (montane) communities are widely distributed in the UK, occurring wherever local edaphic (soil), drainage or relief factors restrict the growth of deciduous forest.

It should also be noted that human activity over thousands of years has cleared almost all of the UK's primary forest growth. Most has been replaced with farmland and where deciduous forest is still present it is often secondary growth.

How might climate change effect ecosystem distribution in the UK?

Deciduous woodlands of southern England may be most susceptible to climate change because of drier summer conditions, which might potentially begin to restrict their range in a high-impact global warming scenario (with temperature rising between 2C and 4C). However, it may also be argued that increased CO2 and a longer thermal growing season will continue to favour the growth of broad-leaved trees. Trees were not always adversely effected by the hot summers of 1976 and 2006 where water remained available. The impact of higher temperatures on trees varies from place to place according to relief and drainage factors.

Where groundwater remains available, there is no reason why trees will not continue to dominate, even if summer drought becomes a permanent climatic feature .However, in areas of thin soils or poor water retention, trees may die and shrub and grasses could be expected to dominate. The net result might be that forest distribution patterns could become more complex, with much denser forest growth close to rivers and more gaps in the canopy wherever soils, geology and relief offer low potential for water storage.

Some scientists also predict an increase in the acreage of coniferous forest in the UK, as the tree-line will move upwards into areas of high relief (e.g. Scottish Highlands). However, the distribution of montane (alpine) ecosystems would be adversely effected at very high altitude as forest growth extends into areas where the growing season has lengthened. As a result, montane communities will be lost as a result of competition from tree species

Student Practice Question:

Outline the factors influencing the distribution of a named biome.

If deciduous forest is chosen as the named biome, then a good answer might refer to climate change as a factor that may already be modifying the existing distribution pattern in drier parts of the UK (e.g. southern England).

Biodiversity in the UK

Biodiversity describes the number of species of living organism within an ecosystem. Some biomes, notably the tropical rainforest, have much greater biodiversity than others (such as Arctic tundra or hot deserts). The climatic conditions promoting vegetation growth in an area are the primary determinant of how many niches exist in an ecosystem that can be occupied by living organisms.

In the UK's native deciduous forest, four-storey growth (canopy, shrub, field and ground layers) supports relatively high levels of biodiversity (especially for insects, animals and birds). However, lower levels of biodiversity are found in those regions of the UK where acidic grassland, heather moorland or coniferous forest dominate as a result of local conditions.

Rare species are sometimes found only in specific coastal environments such as sand dunes and salt marshes. These areas may be granted protected status as a site of special scientific interest (SSSI). Gardens and other human-maintained ecosystems can have unusually high levels of biodiversity for decorative or commercial reasons (estimates suggests British gardens may be home to as many as 15,000 plant species and varieties).

Biodiversity is often higher in an ecosystem during its earlier stages of development and growth, rather than when it reaches full maturity. This is because many ecological communities have the greatest number of species at an earlier stage of succession. In time, the field layer of flowering plants, ferns and grasses can become partially shaded out by a broad-leaved tree canopy, resulting in a fall-off in species richness due to competition for light.

What impact will climate change have on biodiversity in the UK?

The net impact of climate change on biodiversity is difficult to anticipate. Any loss of drought-stressed trees in exceptionally dry areas might adversely effect some bird and insect species. Yet tree death might actually increase local biodiversity as a greater range of flowering plants, grasses and forbs could survive once the dense shade of trees is removed (only 3% of sunlight reaches the ground beneath a dense broad-leaved canopy).

Several reports of changing biodiversity have emerged in recent years:
Garden owners found that indigenous herbaceous plant species such as phlox and lupin struggled to survive during recent heat-waves in 2004 and 2006. In contrast, exotic species such as yucca and olive thrived in the hot dry conditions experienced by southern England. While overall numbers of cultivated species may not decline, the structure of British garden communities is likely to change if gardeners start to favour plants of Mediterranean or Tropical origin that require lower maintenance.
Almost half of the UK's migratory birds have experienced a severe decline in numbers in recent decades. Scientists believe that this may reflect difficulties the birds face in adapting to changing conditions, both in the UK and in Africa where species such as the swallow spend winter.
Changes in marine biodiversity appear to be resulting from warming surface water temperatures (and associated changes in variables such as wave action and salinity). There have been more sightings of large animals such as whales, sharks and sea turtles in British waters.

Some coastal SSSIs face the greatest actual threat to biodiversity as result of climate change. In some instances, rare species will want to move as sea levels rise or rates of coastal erosion accelerate. However, they will be unable to migrate inland if their nature reserve is surrounded by monoculture (intensive agriculture) or urban settlement. In the long-term SSSIs may need to be enlarged and ultimately linked together to form one long biodiversity corridor as part of a sustainability strategy that will leave coastal ecosystems better placed to adapt and change in response to future climate change.

Student Practice Question:

Explain why species diversity can vary from place-to-place and from time-to-time.

In addition to explaining why different types of biome exist in different parts of the world, a good answer will volunteer ways in which climate change may now be effecting the biodiversity of UK ecosystems.

Soil types and distribution

The soil is a natural body of weathered minerals and organic matter differentiated into horizons of variable depth which differ from the material above and below in terms of their physical make-up, chemical composition and biological character. Soil types are a product of the dominant soil-forming processes operating in a given environment. Climate is the key control over soil type at a global scale (although factors such as drainage, parent material and relief can sometimes dominate at the local scale to produce intrazonal soils).

For instance, if precipitation levels exceed potential rates of evapo-transpiration then soil minerals will be dissolved by infiltrating water and carried away in solution via through-flow. The resulting acidified soils found in such a climatic regime are called pedalfers. They develop wherever downward and lateral movements of soil water encourage the processes of leaching, eluviation and occasionally podsolisation (an extreme form of leaching). The UK is a region where the dominant zonal soil type is a type of pedalfer known as a brown earth.

In some areas of the UK, impermeable rock restricts groundwater flow and promotes water-logging. This is especially common in areas of high altitude and/or low gradient in north-western Scotland where evaporation rates are lower and rainfall higher than in southern England. Anaerobic (oxygen-depleted) conditions result, allowing gley soils to develop. This is an example of intrazonal soil formation - podzol development in excessively leached free-draining areas is another.

The brown earth can be expected to remain the domniant zonal soil type for most of the British Isles, barring an extremely high-impact global warming scenario. However, extensive changes can still be anticipated in the pattern of intrazonal soils for the UK if temperatures rise by just 1-2C (a low-impact warming scenario) and rainfall patterns continue their present trend towards greater seasonal variability.

Some gley soils, especially in areas of southern England, might come to resemble brown earths if evaporation rates rise and levels of rainfall-fed infiltration fall during summer months. This seems likely if, as is predicted by some scientists, summers continue to get drier and winter rainfall becomes concentrated in more sporadic heavy downfalls which generate large amounts of surface run-off (thereby limiting the proportion of precipitation that effectively enters the soil). In some areas, seasonal gleying may become the norm, resulting in mottled soil with both blue-grey and reddish-brown colouration on account of both ferric and ferrous iron being present.

Warming could also reduce the extent of podzolic soils in lower altitude areas, given that higher evaporation rates would reduce levels of leaching, eluviation and podzolisation. However, higher rainfall in the north of the UK and Wales (some estimates suggest a 10% annual increase in these areas by 2050) might offset any such soil water losses, thereby preventing too much change.

In a more extreme global warming scenario of 2-3C of more, it is possible that parts of south eastern England might become dominated by pedocal soils, rather than pedalfer soils. Pedocal soils result wherever precipitation levels are lower than evapo-transpiration rates. Under such conditions, soil water can be drawn surface-wards by capillary action, enriching the upper horizons of the soil with previously dissolved minerals. Pedocal soils include the chernozem and chestnut brown soils which currently dominate in drier continental areas of Europe and Asia found at similar latitudes to the UK. High-impact climate change scenarios describe an environment in southern parts of the UK where capillary action would certainly be the dominant process throughout summer months, with only mild winter leaching to offset this.

Student Practice Question:

Describe and explain the pattern of soil types in an area you have studied.

A UK-focused answer might try to apply climate change as a factor that could account for a changing pattern in future years. A good response to this question might suggest that the current soil distribution pattern is becoming more dynamic and will change over time, with pedocal soils becoming more common in the driest areas.

Fieldwork and practical investigations

Themes for geographical fieldwork that link climate with vegetation and soils might include:

Investigating or researching the damage to habitats resulting from an immediate disastrous weather event, such as a hurricane or severe storm, as well as any subsequent additional hazard (such as localised river flooding in Boscastle in 2004).

Analysing a longer term trend or condition, such as the biological impact of a heat-wave or drought (e.g. 1976 or 2006).

While the exact connection between such events and climate change are still not fully understood, there is an emerging consensus that such ‘extreme’ events are connected to global warming. For instance, over the last fifty years, we have witnessed a decrease in the return period of severe droughts, while a higher proportion of UK rainfall now falls on "heavy rainfall days".

Synoptic links that could be explored as part of a fieldwork exercise could include a social and economic impact survey of the biological stresses caused by the increased occurrence of extreme weather events. The impact of hosepipe bans on urban ecosystems such as private gardens, garden centres, public parks and golf courses could all be investigated - with an appropriate focus on consequent loss of livelihood, production and tourist revenues. Questionnaires conducted with small and medium-sized businesses could be a useful technique to employ in a study focused upon the costs and benefits of climatic fluctuations in recent years.

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